The present invention relates to a telemetry system for use in borehole logging tools. In particular, the invention relates to a system for communicating between a borehole tool when it is located in the borehole and a surface system. The invention also provides a system for communication between different tools connected to the same surface system while in the borehole.
In the logging of boreholes, one method of making measurements underground comprises connecting one or more tools to a cable connected to a surface system. The tools are then lowered into the borehole by means of the cable and then drawn back to the surface (xe2x80x9cloggedxe2x80x9d) through the borehole while making measurements. The cable, often having multiple conductors (7 conductor xe2x80x9cheptacablexe2x80x9d is common). The conductors of the cable provide power to the tool from the surface and provide a route for electric signals to be passed between the tool and the surface system. These signals are for example, tool control signals which pass from the surface system to the tool, and tool operation signals and data which pass from the tool to the surface system. A schematic view of a prior art telemetry system is shown in FIG. 1. The system shown comprises a digital telemetry module DTM which is typically located at the surface, a cable C, a downhole telemetry cartridge DTC at the head of a tool string which includes a number of downhole tools T1, T2, . . . each containing a respective interface package IP1, IP2, . . . through which they are in communication with the DTC via a fast tool bus FTB. This system is configured to handle data flows in opposite directions, i.e. from the tools, via the respective IPs and FTB, to the DTC and then to the DTM over the cable (xe2x80x9cuplinkxe2x80x9d), and the reverse direction from the DTM to the DTC and tools over the same path (xe2x80x9cdownlinkxe2x80x9d) Since the principal object of the system is to provide a communication path from the tools to the surface so that data acquired by the tools in use can be processed and analysed at the surfaces the protocol used favours the uplink at the cost of the downlink to optimise data flow from the tools. The communication path is split into two parts, the cable C and the tool bus FTB, and operation of these two are asynchronous to each other. In the FTB, the uplink and downlink both comprise biphase modulation using a half duplex systems of identical instantaneous data rate and frequency synchronised to a clock in the DTC. The difference between the uplink and the downlink is that the uplink uses CRC error detection with retransmission of detected bad packets while the downlink always sends twice. On the other hand, in the cable C the uplink and downlink systems are quite different. Uplink communication uses quadrature amplitude modulation with T5 and T7 cable modes being used, whereas downlink uses biphase modulation and T5 cable mode only. Both uplink and downlink are half duplex with CRC error detection and retransmission of detected bad packets. The result of this is that the uplink will often have an effective data rate of 500 Kbps compared to an effective downlink rate of 40 Kbps. Thus, when running at 6 Hz, such a system will have a period of 166.7 ms of which approximately 150 ms is allocated to uplink. A suitable protocol for implementing such a system is described in U.S. Pat. Nos. 5,191,326 and 5,331,318, the contents of which are incorporated herein by reference.
Recent developments in downhole tools have resulted in tools including functional components which can be accessed via the tool telemetry system and reprogrammed. An example of this is described in WO 97/28466. While it is possible to effect reprogramming of such a tool using the telemetry system described above, the relatively large amount of data to be transmitted downhole and the relatively low data rate of the downlink mean that the amount of time taken to achieve reprogramming of a single tool is likely to be in the order of hours. This effectively means that downhole reprogramming is impractical and even reprogramming at the surface is a slow and intensive process.
The present invention has the object of providing a telemetry system which maintains the priority given to uplink data flow in logging use, but which can be configured to allow increased downlink data flow when required.
One aspect of the present invention provides a borehole telemetry system comprising a surface telemetry module, a downhole telemetry module, and a multiplexed datalink between the surface and downhole modules capable of transferring data alternately between an uplink in which date is transferred from the downhole module to the surface module and a downlink in which data is transferred from the surface module to the downhole module; wherein the data link can be switched between a first configuration in which a relatively long time is assigned to the uplink and a relatively short time is assigned to the downlink, and a second configuration in which a relatively long time is assigned to the downlink and a relatively short time is assigned to the uplink.
The modulation of the uplink and the downlink can be the same or different. In a preferred embodiment, the uplink uses quadrature amplitude modulation and the downlink uses biphase modulation. Where a multiconductor cable is used to provide the datalink different modes can be used for uplink and downlink. For example, T5 and T7 modes can be used for uplink and T5 for downlink. Other modes or forms of datalink can be used if appropriate.
The system can effect the change between the configurations by sending a control signal from the surface module to the downhole module.
A system according to the invention finds particular application in borehole logging using a string of downhole tools. In the first configuration, the system is optimised to send data from the downhole tools to the surface, and in the second configuration is optimised to allow programming of the tools in the tool string from the surface telemetry module. A wireline cable is a common form for the datalink between the surface and downhole telemetry modules.